A speed control apparatus of a motor rotates a motor and a load at a commanded speed given from a host controller by generating torque for accelerating or decelerating a speed of the motor according to a speed error.
Generally, as a value of a speed gain Kv multiplied by a speed error resulting in a difference between a speed command signal and a speed detection signal of a motor by speed control unit is set large, a speed of the motor is corrected at large acceleration with respect to the speed error, so that the speed error decreases in a short time and speed control with high accuracy can be performed.
However, when rigidity of a motor shaft for transmitting torque generated from the motor to a load is low, the value of the speed gain Kv cannot be increased and control accuracy becomes worse. That is, when the rigidity of the shaft is low, the shaft acts as a spring, so that mechanical vibration tends to occur. As a result of this, when the speed gain Kv is set large, the mechanical vibration increases with time and a control system may oscillate.
As unit for solving such a problem, a speed control apparatus of a motor in which a notch filter is inserted into a speed loop of the motor is disclosed in a publication of JP-A-60-39397.
According to the publication, a filter for eliminating only a particular frequency component from an input signal and producing an output is inserted into the speed loop and a frequency component of mechanical resonance is eliminated and thereby oscillation of a control system can be suppressed.
According to such a speed control apparatus of the motor, while suppressing the oscillation of the control system, a speed gain Kv can be set large and as a result, control with high accuracy can be achieved.
However, the motor speed control apparatus as described above is constructed so that resonance of a mechanical system does not occur by setting the notch filter so as to match a notch frequency with a known mechanical system resonance frequency. Then, in order to see the mechanical system resonance frequency, there are a method by calculation and a method by measurement, but there were problems that complicated calculation or a dedicated measuring device is required, respectively.
Thus, as a simple and easy method for matching the notch frequency with the mechanical system resonance frequency, an adaptive notch filter is proposed. The adaptive notch filter is described in, for example, “Analysis of State Characteristics of Adaptive IIR Digital Notch Filter” (Paper Journal of Institute of Electronics, Information and Communication Engineers, Vol. J81-A, No. 9). A block diagram of a speed control apparatus of a motor using an adaptive notch filter is shown in FIG. 1. In FIG. 1, a speed control apparatus of a motor includes a motor 4 having a shaft 4a while driving a load 2, a speed detection part 8 for computing a speed detection signal of the motor 4 by detecting a rotational angle of the motor 4 by an encoder 6 to differentiate the rotational angle (position), a speed control part 12 for obtaining a speed error resulting in a difference between a speed command signal and the speed detection signal by a speed comparator 10 and multiplying the speed error by a speed gain Kv and outputting a source current command signal Ia, a notch filter 14 for generating a compensating current command signal Ic in which a notch frequency resulting in a particular frequency component of the source current command signal Ia is eliminated, a notch filter adaptive part 16 for adjusting a notch frequency so that a current command signal in which a sustained vibration frequency component is eliminated is generated from the notch filter 14, and a current control part 18 for generating a torque command signal of the motor 4 based on the compensating current command signal (current command signal) Ic.
Incidentally, an adaptive notch filter 20 includes the notch filter adaptive part 16 and the notch filter 14.
Contents of processing of the notch filter adaptive part 16 are described in the paper journal and basically, a notch frequency is adjusted so that an output of a ripple of the notch filter 14 decreases. When a control system becomes an oscillation state and the motor 4 and the load 2 vibrate at a resonance frequency, the vibration is detected by the encoder 6 and is inputted to the notch filter 14 through the speed detection part 8, the comparator 10 and the speed control part 12. When the notch frequency does not match with the resonance frequency of the control system, the notch filter 14 cannot suppress oscillation, so that mechanical resonance increases with time. As a result of that, a signal component of the resonance frequency is largely included in input of the notch filter 14 and becomes predominant. In the notch filter adaptive part 16, the signal component of the resonance frequency is largely included in the source current command signal Ia resulting in input of the notch filter 14, so that elimination of the frequency component leads to a decrease in output. As a result, the notch filter adaptive part 16 acts so as to approximate the notch frequency to the resonance frequency and finally, the notch frequency substantially matches with the resonance frequency.
The fact that oscillation of the control system is suppressed by the adaptive notch filter 20 will be described by simulation shown in FIGS. 2 to 4. FIG. 2 is a time chart of the speed control apparatus of the motor shown in FIG. 1 and (a) is motor speeds detected by the speed detection part 8 and (b) is inputs of the notch filter 14 and (c) is changes in a notch frequency adjusted by the notch filter adaptive part and (d) is current commands which are outputs of the notch filter 914 and FIG. 3 is enlarged diagrams of FIGS. 2(a) and 2(b), and FIG. 4 is a gain diagram of a speed control system of the speed control apparatus shown in FIG. 1.
An open loop gain characteristic of a speed control system at the time when a notch filter is not inserted is shown in FIG. 4(a), and a resonance frequency is 1500 Hz and the resonance peak is larger than 0 decibel, so that this state indicates that a control system oscillates.
Therefore, in an initial state, a notch frequency is set to 3000 Hz and using 3000 Hz as an initial value, the notch filter adaptive part 16 adjusts the notch frequency. Control is started from the time 0 and first, a resonance frequency does not match with a notch frequency, so that a control system becomes an oscillation state and a motor speed (speed detection signal) vibrates and the amplitude increases with time. In FIG. 2(a), the motor speed is blacked out and this is because a frequency of the vibration is high. In the case of being enlarged in a time axis direction, vibration occurs at a resonance frequency of 1500 Hz as shown in FIG. 3(a). A speed detection signal from the speed detection part 8 is inputted to the notch filter 14 through the speed comparator 10 and the speed control part 12. A source current command signal Ia inputted to the notch filter 14 is expressed as shown in FIG. 2(b) in the case of enlarging this in a time axis direction, the source current command signal is a vibration component of 1500 Hz as shown in FIG. 3(b). As vibration of 1500 Hz increases, the notch filter adaptive part 16 adjusts the notch frequency so as to eliminate the frequency component and as shown in FIG. 2(c), the notch frequency approximates to 1500 Hz of the resonance frequency with time. When the notch frequency approximates to the resonance frequency at the time (0.1 second), oscillation of the control system proceeds to convergence and the oscillation state is eliminated at the time (0.17 second). Thus, in the adaptive notch filter 20, the notch frequency automatically changes and oscillation is suppressed when the control system starts to oscillate.
However, in the case of well observing FIG. 2(c), it is found that the notch frequency after adjustment is 1770 Hz and does not match with the resonance frequency of 1500 Hz. This is because oscillation converges as adjustment of the notch frequency proceeds and the notch frequency approximates to the resonance frequency to a certain extent.
As described above, in the case that adjustment of the notch frequency functions effectively, a signal component of the resonance frequency must continue to be inputted to the notch filter 14. However, when oscillation of the control system converges, vibration of the motor 4 also stops and the signal component of the resonance frequency is not included in a speed detection signal from the speed detection part 8. As a result of this, the signal component of the resonance frequency is not included in a source current command signal Ia resulting in input of the notch filter 14 and adjustment of the notch frequency by the notch filter adaptive part 16, that is, an adaptive operation does not proceed further. Finally, a notch frequency of 1770 Hz is obtained and a gain characteristic of the notch filter 14 at 1770 Hz is shown in FIG. 4(b) and a gain characteristic of a speed control system including the notch filter 14 is shown in FIG. 4(c).
As shown in FIG. 4(c), though an oscillation state is not reached since the resonance peak does not exceed 0 decibel, it is found that it is near to the limit at which oscillation occurs since the resonance peak is near to 0 decibel. As a result of this, there was a problem that the control system may again run into an oscillation state even when a characteristic of a mechanical system such as the load 2 only changes slightly due to a change with time etc.